Disk drive devices using various kinds of disks, such as optical disks, magneto-optical disks, flexible magnetic disks, and the like have been known in the art. In particular, hard disk drives (HDDs) have been widely used as storage devices of computers and have been one of indispensable storage devices for current computer systems. Moreover, HDDs have found widespread application to moving image recording/reproducing apparatuses, car navigation systems, cellular phones, and the like, in addition to the computers, due to their outstanding characteristics.
A magnetic disk used in a HDD has multiple concentric data tracks and multiple servo tracks. Each servo track is constituted by a plurality of servo data containing address information. Each data track includes multiple data sectors containing user data. Data sectors are recorded between servo data located discretely in the circumferential direction. A head element portion of a head slider supported by a swinging actuator accesses a desired data sector in accordance with address information in the servo data to write data to and retrieve data from a data sector.
It is important to decrease the clearance between a head element portion flying over a magnetic disk and the magnetic disk in order to improve the recording density of the magnetic disk. To this end, some mechanisms have been proposed that adjust the clearance. One of them has a head slider equipped with a heater which heats the head element portion to adjust the clearance (for example, refer to Japanese Patent Publication No. 2007-220195 “Patent Document 1”). In the present specification, it is called thermal fly-height control (TFC). The TFC supplies the heater with electric current to generate heat and make the head element portion protrude by thermal expansion. This leads to decreasing the clearance between the magnetic disk and the head element portion.
It is preferable that the clearance be small for writing or retrieving data. On the other hand, it is important to avoid contact between the head slider and the magnetic disk caused by a too small clearance. The clearance in a beater off state varies depending on the temperature and the operation mode (read or write) as well as the radial position on the magnetic disk. Therefore, it has been proposed to control heater power depending on the radial position on the magnetic disk (for example, refer to the Patent Document 1).
The conventional technique discloses adjustment of the heater power, taking account of the clearance variation caused by variation in attitude and fly-height of the slider depending on the radial position. However, the inventors have found that what changes depending on the radial position are not only the attitude and the fly-height of the slider. Through researches by the inventors, it has been found that the clearance variation rate by the TFC changes with the radial position. That is, the heater power efficiency (Δnm/mW) on the clearance variation varies with the radial position. Consequently, it is necessary that the TFC incorporate the variation in heater power efficiency in the clearance variation depending on the radial position.